The system beeps while it is powering on. Only you hear beep, it can prove system is on successfully.
But if system is beeping frequently while it is not rebooting, maybe you set up buzzer in alarm mode.
Go to main menu--alarm--motion detection/video blind/video loss/abnormality to uncheck buzzer for every channel.
The system will reboot on every Tuesday 1 am and it will cause a beep. please go to main menu---advanced---automatain and disable auto-reboot if you don't want it.